1. Field of the Invention
The present invention pertains to the coating of glass by means of chemical vapor deposition, and more specifically to sensing the position of a coater by which a coating gas is supplied, relative to the surface of a glass ribbon upon which a coating is to be formed.
2. Description of the Prior Art
In accordance with one highly successful method of coating flat glass, the surface of the glass is contacted while at an elevated temperature by a gas such as a metal vapor which reacts on contacting the hot surface to form a coating on the glass surface. In order to produce a uniform coating, the coating gas is preferably caused to flow substantially parallel to the surface of a moving ribbon of glass to be coated under laminar flow conditions. One method and apparatus for achieving such laminar flow conditions is disclosed in U.S. Pat. No. 4,469,045. As disclosed therein, the coating gas is directed onto a surface to be coated by a coater extending across the upper surface of an advancing ribbon of glass and transverse to the direction of movement of the ribbon. The temperature of the gas or vapor supply and that of the coater are preferably maintained sufficiently high to prevent condensation of the coating gas, but sufficiently low to avoid any substantial decomposition or other deleterious reactions of the coating gas before it reaches the glass surface. In order to achieve a uniform coating of high quality, it is imperative that the temperature and rate of flow of the gaseous coating material be precisely maintained within prescribed parameters.
There is disclosed in U.S. Pat. No. 4,793,282 a method of and apparatus for coating flat glass with a coating formed from a plurality of reactant gases. A coater is positioned above an advancing ribbon of glass maintained at a temperature suitable for formation of a coating. Reactant gases are directed to a plenum formed in the interior of the beam. The gases flow from the plenum through flow distributing mechanisms such as baffle stacks to insure homogenizing and uniform distribution across the width of the glass ribbon. The temperature of the plenum, and thus of the gases therein, is regulated by means of liquid cooling and electric heaters.
The downwardly facing surface of the coater must be positioned closely adjacent the top surface of the glass ribbon, typically on the order of 0.200 inch (5.1 mm) therefrom, for the maintenance of a parallel laminar flow of the coating gas as required for production of a uniform coating on the ribbon. The spacing of the coater from the ribbon surface must also remain constant across the width of the ribbon in order to produce a coating which is uniform across the ribbon. It has been proposed to employ various types of position sensors for determining the distance of the coater from the ribbon surface. However, none of the position sensors employed heretofore has proven entirely satisfactory in that they were not suitable for use over extended periods of time in the hostile environment encountered in the coating area. They also tended to have an adverse effect upon the coating, and the data obtained was unreliable. As a result, the position of the coater relative to the glass surface has generally been determined heretofore by visual observation and estimation. Such imprecise procedures are not conducive to formation of a uniform coating on the glass. Furthermore, due to the close proximity of the coater to the glass ribbon, even slight errors in positioning of the coater can result in the coater coming into contact with and severely damaging, it not entirely severing, the ribbon.